1. Field of the Invention
The present invention relates to MOS-technology power devices and their processes of manufacture.
2. Discussion of the Related Art
MOS-technology power devices (such as power MOSFETs and IGBTs) are made up of a plurality of elementary functional units connected in parallel to conduct a fraction of the overall power device current to each unit.
Each elementary functional unit includes a body region of one conductivity type (P type for an N-channel device) formed in a lightly doped semiconductor layer of an opposite conductivity type (N type) common to all the functional units. A heavily doped source region of the opposite conductivity type (N type) is provided inside the body region. The portion of the body region between the source region and the edge of the body region itself is covered by a thin oxide layer, the gate oxide, and by a polysilicon layer, the gate layer, and forms a channel region of the elementary functional unit. The source regions of all the elementary functional units are contacted by a metal layer constituting a source electrode of the power device. Typically, the body regions have a square plan, and the power device is made up of a bidimensional array of square-plan elementary cells. Another conventional layout provides for hexagonal-plan elementary cells.
Typically, each elementary cell has a parasitic vertical bipolar junction transistor (BJT) having emitter, base and collector respectively represented by the source region, the portion of the body region under the source region, and the lightly doped semiconductor layer (an NPN transistor in the case of an N-channel power device). For a correct working of the power device, it is often important to prevent such a parasitic transistor from being triggered on. In the case of a power MOSFET, the parasitic transistor can lower the breakdown voltage of the power device. In the case of an IGBT, two complementary parasitic bipolar transistors are present, forming a parasitic SCR which, if triggered on, can cause the destruction of the power device.
A common technique used to prevent the triggering on of the parasitic bipolar transistor associated with the source and body regions provides for short-circuiting them in each elementary cell. The base-emitter junction of the parasitic transistor is thus short-circuited. In this condition the parasitic transistor breakdown voltage is BVCES (breakdown voltage between collector and emitter with the base short-circuited to the emitter).
Usually, the body region has a central heavily doped region (sometimes called the deep body region) and a lateral lightly doped channel region. The source region has an annular shape and is formed in the lateral channel region, substantially around the deep body region. The source metal layer which contacts the source region also contacts the deep body region of all the elementary cells.
A conventional manufacturing process includes: epitaxially growing a lightly doped semiconductor layer of a first, e.g. N, conductivity type over a heavily doped substrate of the N conductivity type, in the case of a power MOSFET, or of the P conductivity type, in the case of an IGBT; implanting, by use of a mask, and diffusing a heavy dose of a P type dopant into selected regions of the N type layer, to form the deep body regions of the elementary cells; thermally growing a thin oxide layer over the surface of the N type layer; depositing a polysilicon layer over the thin oxide layer; selectively etching the polysilicon layer around the deep body regions; implanting a low dose of a P type dopant using the polysilicon layer as a mask; diffusing the P type dopant to form channel regions extending under the thin oxide layer; and implanting, by use of a mask, a heavy dose of an N type dopant into the deep body and channel regions of the cells to form annular source regions.
The doping level of the deep body regions must be suitable to obtain low-resistance metal-semiconductor contact regions for the body regions, while the doping level of the channel regions is adjusted on the basis of the threshold voltage value desired for the power device. Typical implantation doses are in the range of 10.sup.13 -10.sup.14 atoms/cm.sup.2 for the channel regions, and 10.sup.15 atoms/cm.sup.2 for the deep body regions. Three distinct masks are necessary to introduce the dopants for the various regions of the elementary cells. The windows opened in the implantation mask for the deep body regions are smaller than the windows opened by etching in the polysilicon layer to avoid lateral diffusion of the heavily doped deep body regions during the thermal steps which can alter the doping profile of the channel regions. The source regions are implanted in a self-aligned way with the edges of the windows in the polysilicon layer, but the implantation mask must further provide unexposed surface regions in the middle of the deep body regions.
Often, in the elementary cell, the source region extends a significant distance into the lightly doped portion of the body region. Since the portion of the body region near the channel region under the source region has a relatively high sheet resistance (in the range of 600 ohm/square), the short-circuit of the base-emitter junction of the parasitic bipolar transistor becomes less effective as the lateral distance from the deep body region increases (a resistor is introduced between the emitter and the base regions of the parasitic transistor, and its breakdown voltage lowers to BV.sub.CER, the breakdown voltage between collector and emitter with a resistor R connected between base and emitter). It is therefore desirable to make the deep body region extend as far as possible under the source region. However, due to the lateral diffusion of the deep body region dopants during the thermal steps of the fabrication process, the dopant profile in the channel region may be altered and the threshold voltage of the power device may not be readily controlled. Thus, in current MOS-technology power devices, the region near the edge of the source region to the channel region of the elementary cells is critical from the point of view of the parasitic transistor triggering on.